Donald G. Shomette, a maritime historian and underwater archaeologist, climbed out of the dive boat into the the chest-deep water of St. Leonard's Creek in Southern Maryland to plant the anchor, only to find himself unexpectantly standing on wooden boards.

It was a discovery to dive for, one to probe for with magnetometers, with hydrojets, with infrared aerial imaging or other high-tech devices used in marine archaeology. But there it was, underfoot, discovered by accident, by no more than tactile serendipity.

Shomette and Susan Langley, the state's underwater archaeologist, soon would confirm that this was no mere chunk of river debris, no abandoned duck blind in the muck of the Patuxent River tributary, but an honest-to-goodness shipwreck of a certain age.

The age was Revolutionary or post-Revolutionary, perhaps early 19th century, which is precisely the depths of time that Shomette and company were plumbing this month on St. Leonard's Creek and upriver on the Patuxent. This was the long-awaited search for the Chesapeake Bat flotilla of U.S. Commodore Joshua Barney, scuttled to keep it from the British during the War of 1812.

Barney's flotilla of two gunboats, a sloop-masted flagship, 18 oar- and sail-powered barges-of-war and three other vessles was assembled hastily to ward of British naval attacks in Tidewater. Although it failed to keep the British from burning Washington, it tied up the Royal Fleet for four months, a defensive victory of sorts.

From the lower Patuxent, the British bottled up Barney's fleet until June 26, 1814, when the American commodore blasted his way out of the blockade. Two sluggish gunboats and several merchantmen were left behind and scuttled. Less than a month later, the rest of the fleet was set ablaze upriver, near Upper Marlboro.

Shomette, 52, who is retired from the Library of Congress, first probed for the fleet with a metal-detecting magnetometer in 1979. He found the flagship Scorpion nearly intact under sediment the next Summer. But his dream of locating and even excavating a vessel or two languished for lack of money until this year, when the state contributed $27,000 and several agencies signed on to the project.

Shomette and his contingent of a dozen or so underwater explorers, including a professor and students from East Carolina University, started work June 30 and finished last week. Their efforts proceeded in the face of thunderstorms and heat waves --- and a roller coaster of apparent finds, disappointments and real finds.

A hoped-for gunboat the explorers thought they'd found in a sheltered cove turned out to be a bugeye, a Chesapeake Bay oyster dredgeboat, that was being converted from sail to power. Still, they recovered a cooler full of artifacts, from which they made rubber castings before returning them to the water, until such time as they can be preserved.

The flotilla project has the support of Maryland Gov. Parris N. Glendening (D-Md.). Sen. Paul S. Sarbanes (D-Md.) and Rep. Steny H. Hoyer (D-Md.) are seeking a $214,000 congressional appropriation to continue it. The project is expected to cost millions and take several years to complete, possibly in time for the bicentennial of the war itself.

This summer, those probing the upriver site found the flagship Scorpion buried under three more feet of sediment than in 1980, and, using high-powered hoses known as hydroprobes, located possible sites of more wrecks. But the main action was on St. Leonard's Creek, where divers placed a large boom around the bugeye site and searched up the creek for other finds.

Led by Shomette, they found a prehistoric midden, essentially a garbage pile of oyster shells left by Indians, and they found what they believed to be the wharves of St. Leonard's Town, platted in 1706 and destroyed by the British in 1814.

It was near those two sites that Shomette literally stumbled onto the wreck he believes may be one of the scuttled gunboats from the Chesapeake flotilla.

"Are you excited? Are you happy?" Langley asked Shomette from the deck of the dive boat. "Have we found something good? Might it be a keel?"

Shomette estimated its beam at 20 feet and its length at 70 feet. "This is big," Langley said, joining him in the water. "Whatever it is, this is big."

The next day, the divers-turned-waders had marked off the area with poles sticking out of the water and found mostly wooden and a few metal fittings that confirmed the thing was indeed a shipwreck. They returned again in the final days of this Summer and actually lifted the keel out of the water, but only for a moment.

It had taken three or four months and a public hearing to obtain a permit, allowing Shomette's crew to undertake only the most preliminary of work. Any further excavations, much less retrieval of artifacts, will require another permit, and another season.

"That very likely could be one of the gunboats we know are here," Shomette said. "But we have no permit to dig. We won't know until next year."